Fig. 5 Metronome:
A Metronome is a device used in the music industry. It indicates the ritme by a 'toc-toc' sound which speed can be adjusted with the 250K potentiometer. Very handy if you learning to play music and need to keep the correct rhythm up.Error fixed with thanks to Grant Fair in regards to the two resistors.
(Grant also added a PNP power transistor to increase the volume and a led for visual as well as sound output).